Anaerobic digestion is a series of processes in which microorganisms break down biodegradable material in the absence of oxygen. The process occurs naturally and can take place in many environments including conditions of high pressure. It is widely used to treat organic wastes such as wastewater sludge because it provides volume and mass reduction of the input material. Anaerobic digestion is widely used as a renewable energy source because the process produces a methane and carbon dioxide rich biogas suitable for energy production.
The digestion process begins with bacterial hydrolysis of the input materials in order to break down insoluble organics such as carbohydrates and make them available for other bacterial. Bacteria convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids and then convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. These products are then finally converted to methane and carbon dioxide.
A digester is a process for optimizing the anaerobic digestion of biomass and possibly, for recovering biogas for energy production. Digesters control the environmental conditions of the resident organic material in order to continuously process/digest biomass into a waste sludge of reduced solids concentration and biogas. Anaerobic digesters are made out of many materials. They take the form of silos, troughs, basins or ponds.
All anaerobic digestion system designs incorporated the same basic components:                Inlet sludge supply and conditioning system        Digester vessel(s)        Digester mixing and heating system        Digested sludge removal and disposal system        Biogas removal and conditioning system        
What is needed in the art is a pressurized digester that is a pressurized digester that is considerably more cost effective than the standard open top unpressurized digesters.
The present invention is aimed at three general commercial applications. They are an energy efficient large building such as a condominium having a digester to process wastes and generate electricity; a cattle farm having a digester to reduce animal waste and create electricity to run the farm; a municipal wastewater facility having a large, pressurized digester to reduce waste and produce enough electricity to run itself. A discussion of the energy efficient large building follows below.
Future energy efficient buildings will aim at being free from the use of fossil fuels. FIGS. 1, 1a show a building 1 having mixed use residential/commercial space 5. Clean energy is derived from the building's shape which functions as a solar collector as the earth orbits the sun. Wind energy is derived from a turbine section 4. See Pub. No. US2006/0156725 incorporated herein by reference. Curtain wall material-science technology panels are used throughout wherein the panels reflect solar heat in the summer and transmit solar heat in the winter. All black and grey water exits separate pipes in shaft 7. A black/grey waste processing system 6 includes mixing systems in digesters 8. Oxygen producing plants 3 are incorporated in the terraced roof system that can be a greenhouse. The waste processing system 6 could generate enough electricity to power building 1. The black and grey water shaft 7 enter a single stream recycle waste storage bin set 70. Next, a grey waste clarifier 71 clarifies the grey waste. Next, the settled grey waste is held in bins 72. Next, a sewage clarifier 73 further clarifies the settled grey waste from bins 72. Next, the liquid pretreatment processor 82 readies the slurry for injection into the digesters 8. Output from the digesters 8 includes a water treatment facility 74, and biogas goes to the biogas treatment processor 75. Next the biogas goes to the natural gas pipeline 76 to the facility boilers 77. Steam from the facility boilers 77 powers the electric generators 78. The composter reactor 79 readies the solid waste for removal. A microfiltration unit 80 readies digester water output for delivery to the storage pond 81, which could also hold excess from clarifiers 71, 73.
The main aspect of the present invention is to improve the efficiency of the digesters 8.
Related art includes U.S. Pat. No. 4,690,764 (1987) to Okumura et al. which discloses a horizontal aerator using a pressurized liquid nozzle nested in a gas inlet nozzle assembly, wherein oxygen in small bubbles is mixed into the raw water liquid. A fluid straightener ejects the aerated mixture into a water tank.
U.S. Pat. No. 5,942,116 (1999) discloses an anaerobic sludge digester having an egg shape. A central draft tube has pump to pump settled sludge at the bottom of the vessel up the tube to the top where spray nozzles create motion on the top surface.
The present invention is an apparatus for mixing both homogeneous liquids and non-homogeneous liquid slurries in vessels/reactors of varying geometry. Non-homogeneous slurries include mixtures of liquids, solids and gases in varying relative concentrations. The solids in non-homogeneous liquid slurries include both floating and settling constituents. The apparatus is capable of fully entraining floating and sinking solids for the purpose of promoting a chemical or biological reaction that changes the relative constituency of the slurry components that can vary from inert solids of varying size to complex organic molecules and particles.
Reactor dynamics can include control of slurries that can segregate in a variety of ways. Slurries can include a fully homogeneous liquid mixture entraining non-homogeneous materials. This material includes light solids trapped by rising gas bubbles that form a froth on the surface of the slurry. Materials of both high and low specific gravity tending to float or sink unless acted on by a positive mixing velocity gradient and fibrous and stringy materials. In many cases the ability of a reactor to perform the intended chemical/biological function depends on the operation of a device that blends both floating and settled slurry components with the liquid and neutrally buoyant slurry mass.
Reaction vessels can be built in many configurations. However, tall cylinders present an economical configuration for most reactor vessels that include both low and high-pressure environments. Mixing devices differ in the way they accomplish entrainment of the vessel contents in the three general zones of interest within the reactor. These zones from top to bottom are the surface where slurry components can accumulate as a scum or froth. The center includes suspended slurry components. The bottom where high specific gravity materials settle when mixing gradients are insufficient to maintain the material in the center mix zone.
It can be shown that the energy required of a given mixing system to completely entrain all components of a mixed slurry is greatest for vessel bottoms and surfaces where a range of from low to high specific gravity solids are present. Mixers can be designed either to induce sufficient energy into the slurry in the form of localized slurry velocity to completely mix all zones of the reactor or selectively mix specific zones of the vessel with the intent to direct sufficient energy to accomplish acceptable component-entrainment. The former method uses the largest amount of energy to accomplish mixing, while the latter requires a more complex system to do the work.
A simple mixer design capable of mixing liquid slurry in all zones of a vessel/reactor is needed as an improved, more efficient, more effective and more reliable method to do the work.
The present invention uses a pressurized vessel having a double channel central draft tube in a pressurized and heated vessel. A pump forces fluid through a downward facing nozzle in the central channel. The high velocity fluid induces a vacuum in the outer channel to draw in sludge at various levels of the draft tube. The combined flows from the central and outer channels exit a second nozzle to hit the bottom of the vessel.